Roof insulation/venting in heavy snow load climates
I put the cart before the horse, and selected the house depicted below to build in the Adirondacks, NY. After working out the design details inside the house, I moved to the outside. I questioned the building designer about the low profile 3/12 pitch roof and how it is vented on the high edge, and the reply was "this home uses water impenetrable closed cell urethane insulation, sprayed in to fill the cavity from below prior to drywalling, and it therefore unvented. This insulation (SPF) has an R-value of about 7 per inch, allowing for 2x8 rafters." This sounded great, but the devil on my shoulder said I better look further. FYI, the larger roof is 30' x 30'. This is post and beam construction, 6" thick walls.
I then checked the snow load - it is about 70 to 80 psf. Then I did some searching online, and found a recommendation that at high snow loads, you must have a vented roof to prevent ice damming. The rationale appeared to be "at higher snow loads, thicker coverage of snow insulates the top of the roof, allowing above-freezing temperatures and melting, and thus is more susceptible to ice dams."
So a conundrum has presented itself - SPF has wonderful insulating properties, but in severe cold climates has this limitation. Looking further, a "over-roof" venting system is recommended for high snow load areas. Using hard foam with channels, on top of the deck.
However, with the house/roof in question, this would return me to the problem of venting the high edge of a slab roof. Furthermore, for low pitch (3/12 and lower), a metal roof or something sprayed on (tar, etc) seems to be in order.
I appear to be trying to fit a square peg in a round hole. I can increase the pitch of this slab roof to 4/12 to allow more surface choices and better venting if the over-roof option is added, but it would still have the issue of how to watertight vent the top edge and possibly still be too susceptible to ice damming/leaks in this climate.
Turning to alternatives - if I switch to a home with traditional peaks in the roof, could a 4/12 pitch work in this climate? It seems like venting/ice dams is the biggest issue, but having sufficient pitch to avoid leakage problems is also a concern. I want to avoid 12/12, which of course is standard in the area, because I have lived with a 12/12 with a 20' high ceiling peak for years and am tired of it. I am thinking of a hybrid, using the SPF underneath and the vented "over roof" as described above. It seems this would lend itself more to a 4/12 with a peak, versus a 3/12 slab.
Any constructive advice would be appreciated. There is the question of venting the top edge of a slab roof, venting an SPF sprayed roof at snow loads of 50 psf or higher, and whether the "over-roof" may be a solution at 4/12 in this climate, assuming a proper watertight vent is used. Thanks!
Posted Wed, 07/23/2014 - 19:29
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